Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F11: Nematicity in Febased SuperconductorsFocus

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Sponsoring Units: DMP Chair: Anna Boehmer, Ames Laboratory Room: 307 
Tuesday, March 15, 2016 11:15AM  11:51AM 
F11.00001: Ising nematic quantum critical point in a metal: a Monte Carlo study Invited Speaker: Samuel Lederer The Ising nematic quantum critical point (QCP) associated with the zero temperature transition from a symmetric to a nematic {\it metal} is an exemplar of metallic quantum criticality. We have carried out a minus signfree quantum Monte Carlo study of this QCP for a two dimensional lattice model with sizes up to $24\times 24$ sites. The system remains nonsuperconducting down to the lowest accessible temperatures. The results exhibit critical scaling behavior over the accessible ranges of temperature, (imaginary) time, and distance. This scaling behavior has remarkable similarities with recently measured properties of the Febased superconductors proximate to their putative nematic QCP. [Preview Abstract] 
Tuesday, March 15, 2016 11:51AM  12:03PM 
F11.00002: NMR evidence for inhomogeneous nematic fluctuations in BaFe$_2$(As$_{1x}$P$_x$)$_2$ Adam P. Dioguardi, Tanat Kissikov, ChingHan Lin, Kent R. Shirer, Matthew M. Lawson, HansJoachim Grafe, JiunHaw Chu, Ian R. Fisher, Rafael M. Fernandes, Nicholas J. Curro We present evidence for nuclear spinlattice relaxation driven by glassy nematic fluctuations in isovalent Pdoped BaFe$_2$As$_2$ single crystals. Both the $^{75}$As and $^{31}$P sites exhibit stretchedexponential relaxation similar to the electrondoped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a universal phenomenon in the ironbased superconductors. [Preview Abstract] 
Tuesday, March 15, 2016 12:03PM  12:15PM 
F11.00003: Measuring Nematic Susceptibilities from the Elastoresistivity Tensor A. T. Hristov, M. C. Shapiro, Patrick Hlobil, Akash Maharaj, JiunHaw Chu, Ian Fisher The elastoresistivity tensor $m_{ijkl}$ relates changes in resistivity to the strain on a material. As a fourthrank tensor, it contains considerably more information about the material than the simpler (secondrank) resistivity tensor; in particular, certain elastoresistivity coefficients can be related to thermodynamic susceptibilities and serve as a direct probe of symmetry breaking at a phase transition. The aim of this talk is twofold. First, we enumerate how symmetry both constrains the structure of the elastoresistivity tensor into an easytounderstand form and connects tensor elements to thermodynamic susceptibilities. In the process, we generalize previous studies of elastoresistivity to include the effects of magnetic field. Second, we describe an approach to measuring quantities in the elastoresistivity tensor with a novel transverse measurement, which is immune to relative strain offsets. These techniques are then applied to BaFe2As2 in a proof of principle measurement. [Preview Abstract] 
Tuesday, March 15, 2016 12:15PM  12:27PM 
F11.00004: Origin of the inplane resistivity anisotropy of the iron pnictides: scattering rate or plasma frequency? Michael Schütt, Jörg Schmalian, Rafael Fernandes The prime experimental tool to probe the electronic nematic phase in the iron pnictides is the inplane resistivity anisotropy, which can arise from an anisotropic scattering rate and/or an anisotropic plasma frequency. To shed light on its origin, we investigate the impact of spin fluctuations on the anisotropic ac conductivity of the iron pnictides. We show that two mechanisms contribute to the ac conductivity anisotropy. On the one hand, the inelastic scattering by spin fluctuations directly introduces an anisotropic scattering rate. On the other hand, the same inelastic scattering causes the renormalization of the Fermi velocity at the hot spots. Interestingly, while both mechanisms affect the ac conductivity anisotropy, only the first causes an anisotropy in the dc limit. In contrast, the second mechanism effectively renormalizes both the plasma frequency and the scattering rate. The latter effect opposes the anisotropy induced by the direct scattering of electrons, effectively reducing the observable scattering rate anisotropy. Our results agree qualitatively with recent experiments in detwinned iron pnictides and show the unavoidable entanglement between the scattering rate anisotropy and the plasma frequency anisotropy that arises from spin fluctuations. [Preview Abstract] 
Tuesday, March 15, 2016 12:27PM  12:39PM 
F11.00005: Nematic Crossover in BaFe2As2 under Uniaxial Stress Xiao Ren, Lian Duan, Yuwen Hu, Jiarui Li, Rui Zhang, Huiqian Luo, Pengcheng Dai, Yuan Li The nature of the nematic order in ironbased superconductors has invoked intense research interest. A substantial portion of experimental attempts on resolving this issue required the use of singledomain samples produced under external stress. Here we use Raman scattering, a technique that can detect spontaneous pointgroup symmetry breaking without resorting to singledomain samples, to study BaFe2As2, the parent compound of the “122” Febased superconductors. We show that an applied compression along the FeFe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature Ts into a crossover that spans a considerable temperature range above Ts. Even in crystals that are not subject to any applied force, a distribution of substantial residual stress remains, which may explain phenomena that are seemingly indicative of symmetry breaking above Ts. Our results are consistent with an onset of spontaneous nematicity only below Ts. [Preview Abstract] 
Tuesday, March 15, 2016 12:39PM  12:51PM 
F11.00006: Neutron scattering in detwinned SrFe$_{2}$As$_{2}$ single crystals Li Zhang, Yu Song, Yu Li, Rui Zhang, Weiyi Wang, Haoran Man, Pengcheng Dai \textbf{Abstract}:Large SrFe$_{2}$As$_{2}$ single crystals (2cm) were grown with selfflux method. The basic sample characterizations were described by XRD, MPMS and PPMS. Orthorhombic $a$ along horizontal orientation and $b$ along vertical orientation were determined by Xray Laue diffraction. The crystals were cut into rectangular pieces along the [1,1,0] and [1,1,0] directions by high precision wire saw. The device for sample detwinning was made of 6061 aluminum alloy with low neutron incoherent scattering cross section. Uniaxial pressure can be applied by a spring along orthorhombic [0,1,0] direction by tuning the screw in one end. The pressure can be calculated by the known elasticity coefficient (k $=$ 10.5 N/mm) and the compression of the spring ($\Delta $x) [1]. Our neutron scattering experiments were carried out using the MAPS at the ISIS in England. Low Energy (such as Ei$=$80meV) with different temperatures ,especially around (TN $=$ Ts $=$ 193 K) is done in the timeoffight experiment[2]. It is interesting to find out the pressure induced spin excitation anisotropy. After careful analysis,we conclude that resistivity and spin excitation anisotropies are likely intimately connected. The results also compared with similar experiment in parent BaFe2As2 in Murlin at the ISIS. \textbf{Keywards:} neutron scattering, detwin, SrFe$_{2}$As$_{2}$, single crystals Figure 1, Large SrFe$_{2}$As$_{2}$ single crystals grown with selfflux method. References: [1] Xingye Lu \textit{et al.}, Science 345, 657 (2014). [2] Pengcheng Dai, Rev. Mod. Phys.~87, 855(2015). [Preview Abstract] 
Tuesday, March 15, 2016 12:51PM  1:03PM 
F11.00007: Nematic fluctuations and acoustic phonon in the Raman response of ironbased superconductors M. Khodas, W.L. Zhang, G. Blumberg, P. Richard, H. Ding, Athena S. Sefat Nematicity is a generic feature in the under and optimallydoped ironbased superconductors. Raman and shear modulus studies indicate a critical behavior of the xy symmetry susceptibility towards an extrapolated temperature $\theta$ defining a hidden critical point tens of degrees below the structural transition $T_S$. It was proposed that Raman scattering is insensitive to the orthorhombic lattice deformation and a strong electronphonon (ep) coupling could possibly lift the nematic transition from $\theta$ to $T_S$, while the ep coupling strength remains unknown. Here we report a very low frequency phonon mode associated with an orthorhombic lattice deformation near $T_S$ contributing to the xy symmetry Raman response. We propose an ep coupling model to describe the Raman response and establish the connection of Raman susceptibility and elastic shear modulus. The ep coupling strength deduced from the Raman response is insufficient to lift $T_S$ by 60~K above $\theta$, suggesting that the structural phase transition at $T_S$ and the hidden phase transition at $\theta$ have different origins. [Preview Abstract] 
Tuesday, March 15, 2016 1:03PM  1:15PM 
F11.00008: Electronic Raman study of superconducting Ba$_{1x}$K$_x$Fe$_2$As$_2$ Shangfei Wu, Girsh Blumberg, Pierre Richard, Hong Ding, Haihu Wen We use electronic Raman scattering to probe the superconductivity gap structure and collective modes in underdoped and optimallydoped Ba$_{1x}$K$_x$Fe$_2$As$_2$. In the underdoped samples, we observe a sharp superconducting coherence peak at 60 cm$^{1}$ in the XY (B$_{2g}$) geometry below $T_c$, which is consistent with the gap value determined by ARPES on the outer hole Fermi surface pocket \footnote{Nat. Commun. \textbf{2}, 394 (2011)}. The Raman spectrum shows a threshold at approximately 30 cm$^{1}$ followed by the superconducting coherence peak with a lowenergy tail. We identify a peak at around 95 cm$^{1}$ between 13 K and 22 K in the same geometry in the orthorhombic phase below $T_c$, which becomes broader and weaker upon heating. A sharp and symmetric mode at 120 cm$^{1}$ is also observed in the optimallydoped samples in the same geometry. These collective modes have similar energy scale with the neutron spin resonance mode, the kink observed by ARPES and the bosonic mode observed by STM \footnote{Nature \textbf{456}, 930 (2008); PRL \textbf{102}, 047003 (2009); PRL \textbf{108}, 227002 (2012)}, indicating that they have intimate relationship with superconductivity in the iron pnictides. [Preview Abstract] 
Tuesday, March 15, 2016 1:15PM  1:27PM 
F11.00009: Numerical Study of IronBased Superconductors and Nematic Order Harrison Ruiz, Martin Claassen, Yao Wang, Chunjing Jia, Brian Moritz, Thomas Devereaux We perform an exact diagonlization study of the multiorbital Hubbard model including coupling to lattice degrees of freedom.The singleparticle spectral function, dynamical spin, charge, and Raman response are studied to examine a spin nematic phase due to a biquadratic exchange coupling. [Preview Abstract] 
Tuesday, March 15, 2016 1:27PM  1:39PM 
F11.00010: The realization of nematic order in ironpnictide superconductors HongYi Chen, ChungPin Chou, C. S. Ting The interplay between the nematicity and superconductivity in ironpnictide is studied with a proposed magnetic configuration in a microscopic model. The spindriven order in the nematic state has been found in a small area in the electrondoped regime. In the nematic state, in the normal state, the broken degeneracy of the orbitals $d_{xz}$ and $d_{yz}$ causes the elliptic Fermi surface. In the state where the nematicity coexists with the superconductivity, an orthorhombic magnetic fluctuations appears and its Fourier transformation shows two uneven pairs of peaks at $(\pm\pi,0)$ and $(0,\pm\pi)$. Finally, two modulated stripe SDW perpendicularly intertwined each other and makes the charge density and the spatial distribution of the LDOS reflcting a $d_{x^2y^2}$symmetry form factor. [Preview Abstract] 
Tuesday, March 15, 2016 1:39PM  1:51PM 
F11.00011: SpinDriven Nematic Instability in Realistic Microscopic Models: Application to IronBased Superconductors Morten Holm Christensen, Jian Kang, Brian M. Andersen, Rafael M. Fernandes Electronic nematicity due to the partial melting of density waves is a prevalent phenomenon in the field of high temperature superconductivity. In contrast to usual electronic instabilities, such as magnetic and charge order, this fluctuationdriven order cannot be captured by the standard RPA method. By including fluctuations beyond RPA, we derive the orbitallyresolved nematic susceptibility of a generic multiorbital Hubbard model, thus putting it on equal footing with other electronic susceptibilities of weakly and moderately interacting systems. Application to ironbased superconductors reveals that the $d_{xy}$orbital plays a primary role in promoting a nematic transition preempting the magnetic transition. It furthermore demonstrates the importance of highenergy magnetic fluctuations in stabilizing nematic order in the absence of magnetic order. Finally, we show that the RPA ferroorbital susceptibility shows no divergence on its own, providing strong evidence for a magnetic mechanism for nematicity. [Preview Abstract] 
Tuesday, March 15, 2016 1:51PM  2:03PM 
F11.00012: Effect of Orbital Nematicity on Superconductivity in the Iron Pnictides and Chalcogenides Andriy Nevidomskyy, Rong Yu Orbital ordering leading to the observed nematic phase in the ironbased superconductors has been firmly established in a variety of experiments. It is therefore important to investigate the effect of the orbital order on the superconductivity. To this end, we have performed strongcoupling calculation within the slaveboson approach to the multiorbital $t$$J_1$$J_2$ models for the ironbased superconductors. We report the phase diagram as a function of both electron/hole doping and the orbital ordering strength. We find that the amplitude of the otherwise dominant $A_{1g}$ ($s\pm$) pairing channel diminishes as the strength of orbital ordering is increased, yielding to the $B_{1g}$ ($d_{x^2y^2}$) pairing channel. This effect is especially pronounced in the electrondoped case, with the $d$wave pairing stabilized by the realistic values of the orbital splitting $\sim 50$ meV. While the $d$wave pairing has not been conclusively observed in the ironbased superconductors, the competition between the $s$ and $d$wave pairing found in the calculations may have ramifications for FeSe, KFe$_2$As$_2$ and K$_x$Fe$_{2y}$Se$_2$. [Preview Abstract] 
Tuesday, March 15, 2016 2:03PM  2:15PM 
F11.00013: Possible surface nematic order in iron pnictides Kok Wee Song, Alexei Koshelev Nematic fluctuations play important role in the physics of the ironbased superconductors. Indications for weak precursor nematic transition has been found in the compound BaAs$_{2x}$P$_x$Fe$_2$$[$1$]$. However, highresolution specificheat measurements did not reveal any bulk transition$[$2$]$. To resolve this controversy, we consider the possibility of the surface nematic transition preceding the bulk transition. We consider the simplest model of two interacting quasitwodimensional electronic bands and explore the freesurface effects on the nematic order. We found that threedimensional effects suppress the bulk nematic order and therefore this order is enhanced near the surface.\\ $[$1$]$Kasahara, S., et al. "Electronic nematicity above the structural and superconducting transition in Ba(As$_{1x}$P$_x$Fe$)_2$." Nature 486.7403 (2012): 382385.\\ $[$2$]$Luo, X., et al. "Antiferromagnetic and nematic phase transitions in Ba(As$_{1x}$P$_x$Fe$)_2$ studied by ac microcalorimetry and SQUID magnetometry." Physical Review B 91.9 (2015): 094512. [Preview Abstract] 
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